Collaboration Agreements - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 29, 2016	Dec. 31, 2015	Jul. 31, 2015	Mar. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment		$ 1,600,000
Minimum annual revenue period							3 years
Amount of certain milestones achieved in account receivable		19,725,000			$ 21,041,000		$ 21,041,000		$ 19,725,000
Revenue recognized under collaboration agreement					4,766,000	$ 1,783,000	12,466,000	$ 3,112,000
Estimated deferred revenue recognized within one year		$ 5,261,000			16,748,000		16,748,000		5,261,000
Collaborative Arrangement | Celgene Corporation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum success-based milestone payments				$ 45,000,000.0
Amount of certain milestones achieved in account receivable										$ 6,000,000.0
Revenue recognized under collaboration agreement					600,000	200,000	2,300,000	1,200,000
Deferred revenue recorded under collaboration agreement					6,100,000		6,100,000
Estimated deferred revenue recognized within one year					1,800,000		$ 1,800,000
Collaborative agreement period							1 year
Collaborative Arrangement | Celgene Corporation | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum success-based milestone payments							$ 17,000,000.0
Collaborative Arrangement | Celgene Corporation | Upfront Payment Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment				$ 5,800,000
Collaborative Arrangement | Merck Sharp & Dohme Corp.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue recorded under collaboration agreement					22,400,000		22,400,000
Collaborative Arrangement | Merck Sharp & Dohme Corp. | Upfront Payment Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment	$ 12,000,000		$ 2,000,000				8,500,000		$ 1,900,000
Collaborative Arrangement | Medivation, Inc. and Astellas Pharma, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized under collaboration agreement					1,500,000	0	3,600,000	0
Deferred revenue recorded under collaboration agreement					10,800,000		$ 10,800,000
Collaborative agreement period							1 year
Collaborative Arrangement | Medivation, Inc. and Astellas Pharma, Inc. | Upfront Payment Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment							$ 6,000,000
New Clinical Collaborative Arrangement [Member] | Merck Sharp & Dohme Corp. | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum success-based milestone payments							12,000,000.0
Clinical Research Collaborative Arrangement | Merck Sharp & Dohme Corp.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized under collaboration agreement					2,200,000	$ 1,600,000	6,100,000	$ 1,800,000
Deferred revenue recorded under collaboration agreement					7,700,000		$ 7,700,000
Collaborative agreement period							1 year
Clinical Research Collaborative Arrangement | Merck Sharp & Dohme Corp. | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum success-based milestone payments							$ 4,000,000.0
Clinical Research Collaborative Arrangement | Medivation, Inc. and Astellas Pharma, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue recorded under collaboration agreement					$ 4,100,000		4,100,000
Clinical Research Collaborative Arrangement | Medivation, Inc. and Astellas Pharma, Inc. | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum success-based milestone payments							6,000,000.0
Development Funding | Medivation, Inc. and Astellas Pharma, Inc. | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum success-based milestone payments							$ 10,000,000.0